The present invention relates to a method for modifying surface properties of a disc-like shaped article of a vinyl chloride-based resin or, more particularly, to a method for improving the surface properties, e.g. antistatic performance, anti-wearing resistance and the like, of a disc-like shaped article such as gramophone records and video disc made of a vinyl chloride-based resin.
As is well known, most of the gramophone records now commercially on the market are shaped with a vinyl chloride-based resin such as a copolymeric resin or vinyl chloride and vinyl acetate. In contrast to the advantages inherent to these vinyl chloride-based resins as a material for gramophone records, the largest defect in these gramophone records is that they are readily charged electrostatically to attract dusts or other tiny particles floating in the air causing remarkable noise generated when the record is played.
In order to reduce the electrostatic charging on the surface of a gramophone record, there have been proposed several methods in which an antistatic agent is incorporated into the resin compound prior to shaping of the record or the surface of a record is coated with an antistatic agent. The former method, i.e. incorporation of an antistatic agent into the resin compound, is not satisfactory because of the relatively low antistatic effect if not to mention the degraded moldability of the resin compound into a record. The latter method is, on the other hand, defective because the antistatic effect obtained by the method is poorly durable and rapidly lost by repeated playing of the gramophone record. In addition, uniform coating with an antistatic agent is hardly obtained in general because of the relatively low affinity of the antistatic agent to the surface of the gramophone record. When a large volume of an organic solvent is added to the coating agent with an object to improve the affinity therebetween, noise generation is increased and the quality of the sound is badly affected in the playing of the record.
The inventors have continued investigations to solve the above problems in the surface properties of gramophone records and proposed a method previously that the antistatic performance of a record can greatly be improved by treating the record with low temperature plasma of a low pressure gas.
This method of plasma treatment is indeed effective but still has a problem in the non-uniform improvement of the surface properties all over the surface of a record. That is, the antistatic effect obtained in the plasma treatment by holding the record between a set of electrodes for plasma generation is not uniform and considerably differ from portion to portion on the surface of the record. When the extent of the plasma treatment is increased so far not to leave a portion of insufficient effect of plasma treatment, certain portions have unavoidably received excessive effects of the plasma treatment leading to rather increased noise generation contrary to the object. Further, the antistatic effects on both sides of a gramophone record usually cannot be equal in the conventional procedure of the plasma treatment.